1. Field of the Invention
This invention relates to the field of bipolar junction transistor (BJT) structures, and particularly to BJT structures which reduce base-collector capacitance.
2. Description of the Related Art
The RF performance of a BJT is determined by the minority carrier transit times through the base and collector, and the charging times associated with the base-emitter, base-collector, and parasitic diode junctions. Since the transit times are easily improved by thinning the base and collector layers or by changing their composition, much effort has been devoted to the reduction of the charging times. This is particularly important for devices designed for operation at very high frequencies, such as those based on compound semiconductors such as indium phosphide (InP).
The charging times are determined by the capacitance present at each junction. For a compound semiconductor-based transistor, the base-emitter and parasitic capacitances are already reasonably low. Therefore, reducing base-collector capacitance has been a primary focus. A conventional “triple-mesa” BJT structure is shown in FIG. 1 to illustrate the problem of base-collector capacitance. A substrate 10 supports a subcollector layer 12, which in turn supports a collector layer 14, a base layer 16, and an emitter layer 18. One or more metal contacts 20 are formed on subcollector 12 to provide a collector terminal, metal contacts 22 are formed on base 18 to provide a base terminal, and a metal contact 24 on emitter 18 provides an emitter terminal. A base-collector capacitance Cbc exists below base contacts 22, the value of which increases with the amount of base contact area that is directly above collector 14.
There are typically a set of rules which govern the fabrication process used to produce a device such as that shown in FIG. 1. These rules often mandate certain minimum distances between device features, which are needed to ensure the fabrication of a functional, reliable device. For example, a minimum distance “A” might be required between emitter 18 and base contact 22 (referred to as the “ledge width”), and a minimum distance “B” might be specified between base contact 22 and the edge of the base. These rules result in there being a significant base contact area above collector 14, and thus a significant base-collector capacitance Cbc.
One technique for reducing base-collector capacitance is the “collector undercut”, in which a portion of the collector 14 beneath the base is etched away, as illustrated in FIG. 2. Because the dielectric constant of air is much lower than that of the collector material, the undercut has the effect of reducing base-collector capacitance. However, the undercut leaves the base and/or base contact cantilevered over the resulting empty space, leaving the overhanging edges of the base susceptible to breakage. Subsequent processing steps may also break the base cantilever. Such damage is likely to render the device useless.